The present invention relates to a covered solar cell, as well as a manufacturing method thereof, which enables protection from low energy protons that cause deterioration of electrical characteristics.
In a space covered solar cell to be used as power supply for artificial satellites (shown in FIG. 8), an about 50 xcexcm-1 mm thick cover glass 2 is bonded on top of a solar cell 1 with silicon adhesive 3. Radiations of various energies are flying across the cosmic space. The solar cell 1, when receiving such radiation impinging thereon, suffers crystal defects so that its photoelectric conversion capability is deteriorated. In particular, protons of low energy, upon collision against an object, are absorbed by the very surface without reaching the interior of the object. However, the solar cell 1 has a PN junction at as shallow a portion of its surface as 0.1 xcexcm-0.3 xcexcm, thus resulting in a large deterioration due to collisions of low energy protons.
For this reason, as described above, the about 50 xcexcm-1 mm thick cover glass 2 is bonded on the surface of the space solar cell with the silicon adhesive 3. With the thusly bonded cover glass 2, low energy protons are absorbed by the cover glass 2, and do not reach the solar cell 1. In this way, radiation deterioration of the solar cell 1 is partly prevented by the cover glass 2. In the case of ordinary glasses, since the glass is colored by radiation, cerium (Ce) is added to the glass to prevent the coloring due to radiation.
Surface electrodes 4 of the solar cell 1 are linearly formed with spacings of 0.5 mm- a few mm so as to efficiently take out electric current from a PN junction portion 5, and so designed as to gather at a current takeout portion (not shown). The surface electrodes 4 are formed of silver or other metals having a low resistivity so that the electrical resistance becomes low. There is a further demand for forming the surface electrodes 4 thick to thereby increase the cross section and decrease the electrical resistance. Whereas the surface electrodes 4 are generally formed by vacuum deposition or the like, forming the electrodes thick for lower electrical resistance would take longer time. Thus, a method of increasing the thickness of the surface electrodes 4 by plating is conceivable.
However, the surface electrodes 4, if formed by plating, would be upsized not only in the direction of thickness but also in the direction of width, so that solar light incident on the solar cell 1 would be shielded. On this account, in order to form the surface electrodes 4 thick only in the direction of thickness by plating, there arises a need of patterning with a resist having a thickness larger than the plating thickness. Further, since plating a P-type silicon substrate 6 directly with silver would result in insufficient adhesion strength, it is necessary that after the patterning of a metallic material such as titanium (Ti) into the electrode shape, another patterning for plating be performed so as to surround the patterned metallic material.
One manufacturing method for solar cells is a hydrogen ion peeling process capable of obtaining silicon wafers which are thin and uniform in film thickness (Japanese Patent Laid-Open Publication HEI 10-93122). In this hydrogen ion peeling process, hydrogen ions are implanted into a polysilicon ingot or wafer, and a second substrate is bonded to the implantation-side surface of the ingot or wafer. Then, by performing appropriate heat treatment, a silicon substrate on the second substrate side is peeled in a small thickness. Then, by using this thin-film silicon, a power-saving type covered solar cell is manufactured.
These conventional solar cells, however, have the following problems. In the case of the covered solar cell shown in FIG. 8, silicon adhesive is used as the adhesive 3 for bonding the cover glass 2 as described above. This adhesive 3 is a very expensive refined resin in response to the requirement that out gas be emitted under high temperature and high vacuum in the space be less in amount. This leads to a problem of increased cost. Also, the adhesive 3, although having a property of being soft at normal temperature, yet exhibits an abrupt property change at lower temperatures below xe2x88x9280xc2x0 C. as compared with silicon and glass. This causes the solar cell 1 and the cover glass 2 to undergo large thermal stress under very low temperature environments. As a result, such faults as damage of the solar cell 1 and the cover glass 2 or peeling of the adhesive 3 are more likely to occur, as a further problem.
The adhesive 3 may overflow to side faces of the solar cell 1 or to the top of the cover glass 2 during the work of bonding the cover glass 2. Since the silicon adhesive 3, when irradiated with ultraviolet rays, would be deteriorated, it is necessary to remove adhesive 3a that has stuck to the surface of the cover glass 2 or flowed over the side faces as described above. However, the work of removing the adhesive 3a may often cause breaks of the very thin cover glass 2, which is as thin as 50 xcexcm-1 mm, or the solar cell 1, thus requiring handling with great precision.
The alignment at the side wall between the solar cell 1 and the cover glass 2 needs to be performed with a dimensional tolerance of, normally, 0.2 mm or less according to the requirements that the solar cell 1 not be exposed and that the cover glass 2 not largely protrude out of the solar cell 1. Besides, when the surface electrodes 4 are formed by plating as described above, the alignment between the ground metal such as titanium (Ti) and the plating patterning for preventing lateral expansion of the surface electrodes 4 needs to be done with high precision. However, this alignment between the ground metal and the plating patterning takes long time for manufacture, making it hard to mass produce the space covered solar cell, as a disadvantage.
Furthermore, in the covered solar cell manufacturing method by the hydrogen ion peeling process, because of a small film thickness of the silicon substrate in the resulting covered solar cell, it is necessary to previously bond a second substrate thereto for mechanical reinforcement. This second substrate is given by an electrically conductive metal material, or an insulating material having optical transmittance to at least part of the solar light, for example, by a glass plate or aluminum plate or the like. In such a case, with a glass plate used as the second substrate, the second substrate is exploited as the cover glass during the formation of the covered solar cell. As a result, during the bonding of the second substrate to the silicon wafer, there may occur problems similar to those in the case of bonding the cover glass to the solar cell. Also, use of aluminum or the like as the second substrate leads to a problem that the cost would be increased proportionally to the second substrate.
Accordingly, an object of the present invention is to provide a covered solar cell, as well as a manufacturing method thereof, which eliminates the need for the removal of overflowed adhesive and high precision alignment, and which involves less distortion by heating even under iterative environmental changes in the earth and the space.
In order to achieve the above object, an aspect of the present invention provides a covered solar cell in which a transparent glass layer is formed directly on a surface of a solar cell.
With this constitution, a glass plate for radiation protection is formed directly on the surface of the solar cell without the aid of adhesion. Therefore, the need of an expensive adhesive for bonding the glass is eliminated, the work of removing any overflowed adhesive is eliminated, and alignment faults between the solar cell and the glass plate are eliminated, by which a cost reduction is achieved. Further, thermal stresses on the solar cell and the glass plate due to property changes of the adhesive under low temperatures are eliminated, and thus damage of the solar cell and the glass plate or peeling of the adhesive can be reduced.
In one embodiment, the transparent glass layer is formed by baking powder glass.
With this constitution, a solution of powder glass dissolved with a solvent is applied to the surface of the solar cell and the solvent is volatilized, by which the powder glass layer is formed simply and directly on the surface of the solar cell.
In one embodiment, cerium is included in the transparent glass layer.
With this constitution, since cerium is included in the transparent glass layer, the transparent glass layer can be prevented from coloring due to radiation.
In one embodiment, the transparent glass layer is made up by stacking a plurality of glass layers having different refractive indices.
With this constitution, the optical reflectivity of the surface of the transparent glass layer can be reduced by such a setting that the plurality of glass layers making up the transparent glass layer have refractive indices increasing gradually from solar cell side toward outside. Thus, the rate of absorption to the solar light is increased so that the photoelectric conversion efficiency is improved.
In one embodiment, thickness of the transparent glass layer is 50 xcexcm-1000 xcexcm.
With this constitution, protons of low energy, which upon collision against an object, are absorbed by the very surface without reaching interior of the object, are effectively absorbed by the transparent glass layer having a necessary minimum thickness.
In one embodiment, the transparent glass layer is formed except a surface electrode formation region.
With this constitution, since the transparent glass layer is formed except the surface electrode formation region, the transparent glass layer is used as a plated electrode formation pattern in later process of forming the surface electrodes in the surface electrode formation region by plating.
In one embodiment, the transparent glass layer is an adhesion layer for bonding radiation protection glass onto the surface of the solar cell.
With this constitution, the glass for radiation protection is bonded to the surface of the solar cell by a transparent glass layer of the same material as the radiation protection glass. Therefore, the need for expensive silicon adhesive is eliminated in the formation of the covered solar cell. Further, since the transparent glass layer as an adhesion layer is superior in ultraviolet resistance, the work of removing overflowed transparent glass layer can be eliminated, so that a cost reduction can be achieved. Further, thermal stresses on the solar cell and the glass plate due to property changes of the adhesion layer under low temperatures are eliminated, and thus damage of the solar cell and the glass plate or peeling of the adhesive is reduced.
One aspect of the present invention provides a method for manufacturing the covered solar cell, comprising a step for forming the transparent glass layer by:
applying a solution of powder glass dissolved with a solvent onto the surface of the solar cell, volatilizing the solvent, and thereafter baking the powder glass.
With this constitution, the transparent glass layer is formed directly on the surface of the solar cell without the aid of adhesion. Thus, the need for expensive adhesive for bonding the radiation protection glass is eliminated, the work of removing overflowed adhesive is eliminated, and alignment faults between the solar cell and the radiation protection glass plate are eliminated, thus allowing a cost reduction to be achieved. Further, thermal stresses on the solar cell and the glass plate due to property changes of the adhesive under low temperatures are eliminated, and thus damage of the solar cell and the glass or peeling of the adhesive is reduced.
One aspect of the present invention provides a method for manufacturing the covered solar cell, comprising the steps of:
forming a PN junction on a surface of a semiconductor wafer;
applying photoresist overall on the surface of the semiconductor wafer, and thereafter performing a patterning so that the photoresist is left only in the surface electrode formation region;
applying a solution of powder glass dissolved with a solvent overall on the surface of the semiconductor wafer, volatilizing the solvent, and thereafter baking the powder glass; and
removing the photoresist and thereafter forming surface electrodes by plating.
With this constitution, since the transparent glass layer is formed except the surface electrode formation region, the transparent glass layer is exploited as a plated electrode formation pattern in the process of forming the surface electrodes in the surface electrode formation region by plating. Further, since no exclusive-use plated electrode formation pattern is required, the need of alignment between the surface electrodes and the plated electrode formation pattern is eliminated, so that the surface electrodes can be formed quite simply.
One aspect of the present invention provides a method for manufacturing the covered solar cell, comprising the steps of:
forming a PN junction on a surface of a semiconductor wafer;
forming a ground metal in the surface electrode formation region on the surface of the semiconductor wafer; and
applying a solution of powder glass dissolved with a solvent overall on the surface of the semiconductor wafer including the ground metal, volatilizing the solvent, and thereafter baking the powder glass.
With this constitution, since the solution is repelled by the ground metal, the transparent glass layer is formed except the surface electrode formation region. Therefore, the transparent glass layer is exploited as a plated electrode formation pattern in the process of forming the surface electrodes on the ground metal by plating. Further, the need for alignment between the ground metal for the surface electrodes and the transparent glass layer is eliminated, so that the surface electrodes are formed quite simply. In addition to this, the need of patterning photoresist for the formation of the transparent glass layer is eliminated, so that the surface electrodes are formed even more simply than in the preceding embodiment of the invention.
One aspect of the present invention provides a method for manufacturing the covered solar cell, comprising the steps of:
applying a solution of powder glass dissolved with a solvent onto a surface of the radiation protection glass;
applying a solution of the powder glass dissolved with a solvent onto a surface of the solar cell;
bonding together the solar cell and the radiation protection glass at surfaces thereof onto which their corresponding solutions have been applied; and
volatilizing the solvent and thereafter baking the powder glass.
With this constitution, the radiation protection glass is bonded to the surface of the solar cell by a transparent glass layer made of the same material as the radiation protection glass. Therefore, no expensive silicon adhesive is required. Further, since the transparent glass layer as an adhesion layer is superior in ultraviolet resistance, the work of removing overflowed transparent glass layer is eliminated so that a cost reduction can be achieved. Furthermore, thermal stresses on the solar cell and the glass plate due to property changes of the adhesive under low temperatures are eliminated, and thus damage of the solar cell and the glass or peeling of the adhesive is reduced.
One aspect of the present invention provides a method for manufacturing a covered solar cell, comprising steps of:
manufacturing a covered solar cell according to the covered solar cell manufacturing method; and
peeling to a specified thickness a rear side surface of the semiconductor wafer in the covered solar cell formed by the preceding step.
With this constitution, in manufacturing a power-saving type covered solar cell by using a thin film semiconductor substrate peeled into a specified thickness on its rear side, the thin film semiconductor substrate is mechanically reinforced by the transparent glass layer. Therefore, the need for bonding an exclusive-use substrate for reinforcing the thin film semiconductor substrate is eliminated, so that problems and cost increases involved in the bonding of the exclusive-use substrate are solved.